There are two processors on the digital Tape Recorder. Both need to be flashed. Be aware: you cannot power the digital Tape Recorder with an FTDI board or ICSP programmer (the 3v3 and 5v lines are not connected). You need to power the digital Tape Recorder (connect to the micro-KIM or a genuine KIM-1) before you can flash the firmware!
The setup for the Arduino IDE is in the main program file.
If you want to use the digital Tape Recorder with a genuine KIM-1 make shure you change
the define _REAL_KIM1 from false to true!
You do need a FTDI board with a RESET and DTR pin. Many FTDI boards have DTR and CTS on the header but you cannot flash the digital Tape Recorder with such a board. Most FTDI board make use of a CH340 chip (or something like that) that do have a RESET pin. It's easy to wire this pin to the CTS pin on the outside (interrupt the CTS track before you solder the RESET line).
But luckely there are a lot of FTDI boards with the RESET pin already connected to the header.
Make sure you have a board that has 3v3 output or you will burn the ESP8266!!
Now connect the digital Tape Recorder to the FTDI board and plug the FTDI board in an USB port.
In the Arduino IDE select the correct Serial port and upload the firmware.
Now you can proceed to the next step.
The setup for the Arduino IDE is also in the main program (ATtinyPLL2).
You need an ISP or ICSP programmer with a 0.2" connector.
Press and hold(!) the RESET (SW1) switch on the digital Tape Recorder and flash the bootloader to the ATtiny841. Next upload the firmware to the ATtiny841.
Your digital Tape Recorder is now good to go!